A wheel hub of a vehicle hub unit generally includes a flange for mounting a brake rotor formed to face radially outward. The flange is provided with bolt holes for mounting the brake rotor. Bolts for mounting the brake rotor are previously press fitted in the bolt holes of the hub unit before the hub unit is mounted on a vehicle drive shaft so that the brake rotor can be attached to the hub unit mounted on the drive shaft. The bolts are then inserted in bolt holes of the brake rotor, and a nut is tightened to a threaded portion of each bolt, so that the brake rotor is secured to the hub unit. Also, each bolt is press fitted in the associated bolt hole of the hub unit by plastically deforming and engaging a serration formed in the vicinity of a head portion of a threaded shaft portion of the bolt in a circular inner circumferential surface of the bolt hole.
By press fitting each bolt while plastically deforming and engaging the serration of the bolt in the circular inner circumferential surface of the bolt hole, however, part of the flange displaced by the plastic deformation bulges on a brake rotor mounting surface of the flange. When such a bulging portion of part of the flange exists on the brake rotor mounting surface, the brake rotor can not be secured to the flange in close contact. As a result, a gap between the brake rotor and a brake pad becomes uneven in the radial direction, which might cause a problem of runout of the brake rotor such as uneven wear of the brake pad.
As a method for solving such a problem, a wheel huh has been proposed in which a widening portion is provided in each bolt hole as disclosed in Japanese Laid-Open Patent Publication No. 2005-54999. FIG. 4 shows the shape of a bolt hole 102 of a flange 101 in a conventional hub unit, and a state in which a bolt 103 is press fitted in the bolt hole 102. As shown in FIG. 4, a first widening portion 105, which is tapered from a bolt seating surface 104 toward the bolt hole 102, is formed at part of the bolt hole 102 close to the bolt seating surface 104 against which a head portion 103a of the bolt 103 abuts. Also, a second widening portion 108, which is tapered from a brake rotor mounting surface 107 toward the bolt hole 102, is formed at part of the bolt hole 102 close to the brake rotor mounting surface 107 on which the brake rotor 106 is mounted. Furthermore, a small diameter portion 109 is formed between the first widening portion 105 and the second widening portion 108. Then, the bolt 103 is press fitted in the bolt hole 102 by plastically deforming and engaging a serration 103b of the bolt 103 in the small diameter portion 109.
Also, the conventional wheel hub 110 includes bolt holes 102 configured as described above in the flange 101, and the bolt holes 102 are machined after finishing the brake rotor mounting surface 107.